Method and apparatus for reducing the water and energy consumption of a paper machine with the help of a vacuum system and optimization of solids content as well as use of the same

ABSTRACT

A method for reducing the water and energy consumption of a paper machine with the help of a vacuum system and optimization of web solids content. On the paper machine wire section, water is removed with the help of a hybrid vacuum system serving first the vacuum locations needing a lower vacuum level and then those requiring a higher vacuum level in such a fashion that the hybrid vacuum system removes water on different sections of the paper machine at the vacuum levels rendering the Individually maximized energy efficiency. Additionally the solids content on the wire section is optimized with fee help of an unfelted and smooth press roll adapted above the wire suction roll.

The invention relates in accordance with the preamble of claim 1 to amethod for reducing the water and energy consumption of a paper machinewith the help of a vacuum system and optimization of solids content.Furthermore, the invention relates to the use of an apparatus inaccordance with the preamble of claim 5 and a hybrid vacuum system inaccordance with claim 10.

As is generally known, a papermaking process is highly energy-intensive.The greatest energy consumers can be basically listed as the heating ofraw materials, pump units at the wet end and the dryer section itself.

In the heating of raw materials for papermaking, fiber and fillers aretaken to the paper mill from outdoors storage. Hence, they must beheated to the process temperature of 40-50° C. The stock is mixed withwater, whose amount typically by weight is 10 to 20 times greater thanthe amount of wood fiber and filler materials in the resultingproduction furnish (furnish consistency in the headbox is generally0.3-1 %, max. 2 %, which translates into 1 part of fiber and similarsolids in 99-300 parts of water). This water volume is recirculated inthe process even if the process does not have a closed circulationsystem. Water consumption per produced ton of paper is about 10-20 m³/t.The necessary amount of water is generally taken from a lake/river at atemperature of 0-25° C., depending on the season of the year. Today, theprocess temperature is elevated up to 40-50° C. in order to improvedewatering drainage. Hence, the effluent water passed from the mill towastewater treatment contains a great amount of low-value thermal energywhose recovery concurrently is unprofitable or complicated.

Additionally, papermaking needs a lot of air that also must be heated.Many mills use large volumes of hot air which is exhausted from theprocess without being utilized by heat recovery.

The above-mentioned great amount of circulation water requires massivepumping power that is another major energy consumer. Eventually, thisenergy is converted into heat thus providing a major portion of thethermal energy required by the process. Drying the paper web on the wireand press sections is partially implemented with the help of vacuum.This portion of the process is known as the process vacuum system. Alsothe vacuum system is a prominent energy user that typically consumes 20%of the mill's overall power.

After passing through the wire and press sections, the solids content ofthe paper web is generally in the range of 40-50%. The final moisturecontent of the paper web is 6-8%. Drying the web after the press takesplace by evaporation. Evaporation of water is a highly intensiveconsumer of energy that is obtained from steam. Hence, the solidscontent of the paper web is advantageously maximized upstream of thedryer section in order to minimize steam consumption.

As said, an important portion of a paper machine is its vacuum system.The vacuum systems of a paper machine can be divided into two basicarrangements: a water ring pump system and a turboblower system.

In FIG. 1 is illustrated a typical paper machine equipped with waterring pumps. The drawing also shows conventional dewatering equipment ofa paper machine that will be discussed later in the text. The systemgenerally comprises 6-15 pcs. pumps that serve the locations of thepaper machine requiring vacuum at different sub-atmospheric pressurelevels. In a water ring pump, the rotating water acts as a piston thatcompresses the entering gas. The compression takes place isothermally,whereby the thermal energy released from compression is mainly absorbedby the seal water passed into the pump. A great amount of seal water isrequired, 100-400 l/min per pump, but 80-90% thereof can be recirculatedif elevation of water temperature can be prevented through cooling theliquid circulation. However, thereby will be lost the thermal energyrecoverable from the compression cycle and also from a portion of theheat released in the process. As the spent seal water contains fiber andimpurities, it is generally passed out from the mill's watercirculation. The overall efficiency of a water ring pump is dependent onthe vacuum level and pump rotation speed. Efficiency at a high vacuum isbetter than at low vacuum.

Alternatively, the vacuum system in paper machines can be implementedwith centrifugal blowers of the type generally known as turboblower. Thevacuum locations in a paper machine are generally operated at a highvacuum in excess of 60 kPa. Hereby one or two multistage blowers arerequired to reach the highest vacuum levels. The number of stages, orimpellers, arranged in series is typically 4 pcs. Generally, at eachimpeller is arranged an intermediate port for connection to lower vacuumlocations. The capacity of this blower type cannot be varied by changingthe speed of rotation. The only possible way of adjustment is bythrottling the air flow. In terms of energy efficiency, this kind ofcapacity control is uneconomical. Additionally, the system also has atleast one single-stage blower for medium vacuum locations. As theexhaust air from the blower system is hot, its thermal energy can berecovered by heat exchangers.

In FIG. 2 is illustrated a typical turboblower system. The pumpingefficiency of a turboblower system is slightly better than that of awater ring pump. On the other hand, when the system has a smaller numberof blowers, the flows to the vacuum locations must be controlled withthrottle valves, whereby the overall efficiency of the system isimpaired. The greatest benefit is appreciated therein that the blowerhas no rotating water ring, i.e., it does not need seal water at all. Incontrast, a significant problem arises from cost of the system due tothe multistage blowers, whose higher price of acquisition andinstallation together with their auxiliary equipment increase theinvestment costs.

In addition to the above systems, the dewatering equipment of a papermachine is a vital element. On the wire section, water is initiallyremoved from the web by gravitation and with foil effects andcentrifugally. Thereupon more differential pressure must be appliedacross the web, since a major portion of water is transferred from thepaper web to the dewatering equipment by compression. Typically,downstream of the felt water level are arranged flat suction boxeshaving a vacuum therein. At the end of the wire section is generallylocated a suction roll. In FIG. 3 is shown the conventional constructionand operating principle of flat suction boxes and wire suction roll.

The vacuum system produces the vacuum in the flat suction boxes and thewire suction roll. Air flows into the suction boxes through the web,whereby a pressure differential is established. This function is highlyenergy-intensive. The pressure differential causes friction between thesuction box and wire thus increasing the energy consumption of the wiredrive motors. Water is removed from the paper web partially along withthe air flow, but also due to caliper compression of the web,particularly for thicker paper grades having a basis weight greater than80 g/m².

The wire suction roll has holes drilled thereto for suction of airthrough the web. The passing-through air does not retard the web travelbut requires more capacity from the vacuum pumps due to the elevatedvacuum level and additional air sucked through the drilled holes. Wateris collected from the web through the wire into the holes of the rollwherefrom it is ejected centrifugally to a water collection pan adaptedabout the roll. In practice has been found that no water will passthrough the roll holes if the web speed exceeds 200 m/min. Anotherpractical experience relates to the crucial roll of the pan as all waterejected from the roll must be collected into the pan, wherefrom it isreturned to the water circulation of the machine. Since the water tendsto adhere to the roll surface and holes due to surface forces, it mustbe separated with the help of different doctoring arrangements.

It must be noted that while no fresh water is needed for the suctionbox, the wire suction roll consumes water by about 100-200 l/min aslubrication for the seal strips of the vacuum chamber of the roll.

Typical solids content after the wire is 15-20%. Solids content ispreferably maximized in order to minimize energy consumption at thelater discussed press and dryer sections, simultaneously achievingimproved machine runnability. A general rule of thumb in the art is thatin paper webs a change of 1% downstream of the wire section results in0.25% increase in solids downstream of the press section.

In the appended FIG. 4 are shown the results of a study performed byPh.D. Räisänen on the effect of vacuum level and dwell time on thesolids content downstream of the wire section. The diagram is reprintedfrom the annual report 1994 of research program Sustainable Paper(Kestävä Paperi). As has also found in practice, the diagram clearlyindicates the optimum running condition, namely, that the vacuum levelmust increase toward the trailing end of the wire travel. Anotherimportant aspect relates to the maximum solids content attainable at agiven pressure differential. As a longer dwell time cannot offer ahigher solids content, a higher vacuum level must be applied. Since avacuum level of about 70 kPa is a practical maximum, compression must beapplied to the web downstream of the wire section in order to reach ahigher solids content. Down-stream of the wire section on the presssection, this function is implemented with the help of presses thatremove water from the web to felts and/or suction roll holes and/orgrooves of a grooved roll. In some paper machines, a press and endlessfelt are adapted above a suction roll. This arrangement typicallyachieves a solids content of about 24% downstream of the wire section.

As elucidated above, running a paper machine involves an extremely greatnumber of factors affecting water and energy consumption. In prior-artarrangements the goal has been to solve one problem at a time. Now thepresent invention attemps to find a solution by way of examining all thedifferent factors separately and then combining their effect in theoverall performance. This approach proved that in the art there stillare substantial possibilities of improvement in the various propertiesof a paper machine. The essential feature of the invention isparticularly a reduced consumption of water and energy by virtue of animproved vacuum system and optimization of web solids content.Resultingly, the invention offers reduced energy consumption in a papermachine through increased solids content downstream of the wire andpress section and, further, by reducing raw water consumption in themill. This goal is attained by utilizing conventional equipment in anentirely novel and innovative fashion and enhancing the operatingpractices of the paper machine. In addition to the economical benefitresulting therefrom, the invention facilitates reduced investment costswith regard to the present situation.

The essential features of the invention are crucial factors in thearrangement defined in the claims. The invention provides pluralsignificant benefits while simultaneously avoiding the problemshampering the prior art.

The arrangement according to the invention implemented using a so-calledhybrid vacuum system, whereby a significant improvement is achieved inthe energy consumption of the paper machine in its front end, on itswire section through reduced water and electricity consumption andincreased web solids content. A significant feature is that theinvention aims to provide a comprehensive improvement of energyconsumption rather than simply attempting enhanced energy use in asingle component such as a pump.

More specifically, the invention is characterized by what is disclosedin the appended claims. The invention is next described in more detailby making reference to the annexed drawings wherein

FIGS. 1-3 show some embodiments of prior-art constructions;

FIG. 4 shows change of solids content as a function of vacuum level anddwell time;

FIG. 6 shows the results of a solids content measurement on adouble-layer wire section; and

FIGS. 5 and 7 show some embodiments of an arrangement according to theinvention.

The above-discussed conventional vacuum system implemented with the helpof turbo flowers does not consume water at all. However, the system ishampered by the costly multistage blowers that are required to achievethe high vacuum levels needed in a paper machine, as well as by theenergy-wasting throttle control. In the embodiment according to theinvention, a so-called hybrid vacuum system 1 is configured, said vacuumsystem being implemented primarily with cost-effective single-stage ortwo-stage blowers 2. The blower impellers are mounted on the motorshaft, which also makes facilitates the capacity control of a possibletwo-stage blower by varying the rotation speed. An essential detailherein is that said blowers are used for serving only those vacuumlocations of the paper machine that can utilize their salient energyefficiency, typically at a vacuum level of 0-60 kPa, namely, all othervacuum locations except wire suction roll 3 and press suction roll 13requiring a higher vacuum. To serve the suction rolls that require anelevated vacuum level, a water ring pump 4 is used that for generationof a high vacuum is a very good and energy-efficient device. No waterseparation is provided between the water ring pump 4 and paper machine5, since only clean lubrication water 6 of the seal strips is pumpedtherein. The seal strip lubrication water travels to the water ring pump4, wherein it is used as seal water 7 of the pump. This water volume,however, is not sufficient to the function of pump 4 as its flow alsovaries according to the running conditions of the paper machine. Hence,a certain amount of supplementary water 8 must be introduced. Since thefunction of pump 4 presumes a correct amount of seal water, the flow ofwater must be controlled in the following fashion: downstream of thepump is installed a water separator 9 whose water volume is measured.This measurement is used to control the amount of supplementary water 8to be delivered to the pump as shown in appended FIG. 5. An alternativemethod of controlling the admission of supplementary water to the pumpis to measure the temperature of the pump 4 that begins to arise if theamount of seal water is insufficient.

As no process water is discharged from the roll 3, the water coming outfrom water separator 9 is clean. The water temperature has risen in thepump and can thus be passed to the process or used as cleaning showerwater that must be warm. Accordingly, the water consumption in thefactory is decreased and its thermal energy can be recovered. Othervacuum locations of the paper machine can be served by speed-controlledturboblowers 10 having single-stage or maximally two-stage construction.Speed control is the most energy-efficient control method, which lowersthe mill's operating costs. The outcome is an especiallyenergy-efficient hybrid system that provides significant savings inwater consumption. By virtue of the hybrid vacuum system, electricityconsumption is typically reduced by about 30%.

The cost of the turboblower 10 is dictated by the number of impellers(i.e., blower stages or steps), as well as by the foundation andauxiliary equipment costs thereof. As to investment costs, the hybridvacuum system is clearly more profitable than prior art systems.Investment costs are about 20-40% lower that in the system illustratedin FIG. 2.

Energy efficiency is also enhanced by way of passing the hot dischargegas exiting from the turboblowers 10 via heat exchangers 11 to theambient. In appended FIG. 5 is illustrated the flow diagram of thehybrid vacuum system 1. Recovered thermal energy is 50-60% ofelectricity input. The recovered heat is used for preheating the intakeair or fresh water inflow of the mill.

In appended FIG. 6 is shown the maximization results of solids contenton the wire section. The measurement has been carried out in a practicaltest performed on a paper machine equipped with another energyutilization enhancement scheme according to the present invention. Thesolids content measurement plotted in FIG. 6 was performed on atwo-layer wire section having 16 pcs. of dewatering elements (on thelower wire). In the diagram is depicted the measurement of solidscontent and water removal on the wire section 12. At the end of wiresection 12, downstream of wire suction roll 3, a solids content of 30%is attained. An essential prerequisite to this end is that the solidscontent shall be about 10% upstream of the wire suction roll 3.

This process behavior can be traced to the configuration in which abovethe wire suction roll 3 is adapted a wire press 18 running unfelted.Rewetting is a common phenomenon known to hamper the operation of afelted press. When the press nip opens, water reflows from the felt backto the web. This rewetting is the stronger the wetter the web.Consequently, a felted wire press cannot reach as high a solids contentas a press having a smooth unfelted roll 18. In the configurationillustrated in FIG. 5, the roll is further mounted at the end portion ofthe vacuum chamber 14 of the wire suction roll. With the help of a waterring pump 4, the roll interior space is kept at a maximized vacuum levelof about 70-75 kPa. The wire press covers a portion of the wire suctionroll 3 thus reducing water flow through the web to the suction roll.However, these arrangements alone are not sufficient for attaining adesired solids content. This requires more effective water removal. Inpractice, when water is transferred from the web or wire to the suctionroll holes 15, the entrained air lands the water as a thin film onto thehole rims. Then, water cannot properly be expelled centrifugally fromthe roll, but rather forms a “mist” about the roll. Instead, when asufficient amount of water is present, the wire press forces the waterinto the roll holes 15 as a plug 16 that readily leaves the rollcentrifugally.

In FIG. 7 is illustrated the principle of water removal as well as thewater film and water plug 16. The most advantageous precondition forforming a water plug is that the roll hole 15 is straight-walled withouta beveled rim. The minimum amount of water can be estimated when theopen surface area of the roll 3 is known. In a practical arrangement,the discharge pipe of the water collection pan of the wire suction rollis provided with a flow meter. By practical tests, a set value isdetermined for the flow. With the help of this set value of water flow,the vacuum level of flat suction box 17 preceding the roll can becontrolled in order to pass a desired amount of water to be removed atthe wire suction roll 3. Then, there is no need to elevate the vacuumlevel of suction boxes unnecessarily, whereby savings are attained inthe energy consumption of the vacuum pump and the wire drive motors.This kind of operation is entirely different from that today used in theart wherein the goal is to maximize the solids content increase at eachwater removal element. The control scheme of the wire suction roll 3 andthe flat suction boxes is illustrated in FIG. 5.

As an additional verification of the concept, the table below proves howthe arrangement according to the invention achieves higher solidscontent down-stream of the press and thus reduces steam consumption onthe dryer section.

As listed below, typical distribution of water removal percentages onthe press section in a three-nip press divides as follows: first press20%, second press 50% and third press 30%. In the table is alsoestimated the increase of solids content for two cases whereindownstream of the wire the solids content is 20% and 30%, respectively(with the assumption that each 1% increase downstream of the wire causesa 0.25% increase downstream of the press).

Results obtained from a paper machine illustrated in FIG. 5 are listedbelow.

Prior art paper Arrangement acc. machine to invention. Wire solidscontent (%) 20 30 Press solids content (%) 46 48.5 Press water removal(l/min) 1000 450 at press #1 200 0 at press #2 500 270 at press #3 300180

As is evident from the table, the arrangement according to the inventionachieves a higher solids content downstream of the press and resultinglyprovides a substantial reduction of steam consumption on the dryersection. A salient feature is that as the solids content increases by10% downstream of the wire, it is possible to obtain a higher solidscontent downstream of the press even if the number of felts is reducedfrom three to two.

The present arrangement offers substantial benefits by optimizing thedifferent individual elements in a novel way into an overall solutiondisclosed in the invention. The essential feature of the invention isthat, on the paper machine wire section 12, water is removed with thehelp of a hybrid vacuum system serving first the vacuum locationsneeding a lower vacuum level and then those requiring a higher vacuumlevel. Additionally, the solids content on the wire section 12 isoptimized with the help of an unfelted press roll 13 adapted above thewire suction roll 3 and finally at the other vacuum locations of thepaper machine in such a fashion that water removal is carried out atdifferent sections of the paper machine with the help of differentvacuum systems running them at their optimal energy efficiency levels.

The hybrid vacuum system is more particularly implemented so thatsingle-stage or two-stage blowers 2 are run at their optimal energyefficiency speed to serve vacuum locations needing a lower vacuum level,typically at a vacuum of 0-60 kPa. Water removal at locations requiringa higher vacuum is carried out with the help of a water ring pump 4 at avacuum level of about 70-75 kPa, while other vacuum locations are servedby speed-controlled turboblowers 10. Simultaneously, the solids contenton the wire section 12 is optimized by mounting above the wire suctionroll an unfelted press roll 13 located at the end portion of the vacuumchamber of the wire vacuum roll. A further essential feature is thatwire press 13 forces water into holes 15 of wire suction roll 3 so as toform water plugs 16 therein thus further increasing the solids content.The overall result is a combined use according to the invention of ahybrid vacuum system and optimization of solids content on the wiresection in order to reduce water and energy consumption on a papermachine.

To a person skilled in the art it is obvious that the invention is notlimited by the above-described exemplary embodiments, but rather may bevaried within the inventive spirit and scope of the appended claims.

1. A method for reducing the water and energy consumption of a papermachine with the help of a vacuum system and optimization of solidscontent, comprising the following step: removing water on the papermachine wire section with the help of a hybrid vacuum system servingfirst the vacuum locations needing a lower vacuum level and then thoserequiring a higher vacuum level in such a fashion that the hybrid vacuumsystem removes water on different sections of the paper machine at thevacuum levels rendering the individually maximized energy efficiency andthe solids content on the wire section is optimized with the help of anunfelted and smooth press roll adapted above the wire suction roll. 2.The method of claim 1, wherein in the method water is removed on thewire section of the paper machine with the help of a hybrid vacuumsystem so that single-stage or two-stage blowers are used to at theiroptimal energy efficiency to serve vacuum locations needing a lowervacuum level and water removal at locations requiring a. higher vacuumIs carried out with the help of a water ring pump, while other vacuumlocations are served by speed-controlled turboblowers and,simultaneously, the solids content on the wire section is optimized bymounting above the wire suction roll an unfelted press roll located atthe end portion of the vacuum chamber of the wire vacuum roll.
 3. Themethod of claim 1, wherein in the method single-stage or two-stageblowers are most advantageously used to serve vacuum locations needing alower vacuum level, typically at a vacuum of 0-60 kPa, while the highervacuum level is elevated to a level maximized with the help of a waterring pump to about 70-75 kPa and, additionally, the wire press forceswater into the holes of wire suction roll so as to form water plugstherein thus further enhancing the solids content.
 4. The method ofclaim 1, wherein in the method the seal strip lubrication water is usedas the seal water of the water ring pump, while the requiredsupplementary water is metered either at a water separator locateddownstream of the water ring pump or by monitoring the pump temperature,whereby the water leaving the water separator is returned to the processor used as the cleaning shower water, and the hot discharge gas exitingfrom the turboblowers is passed via heat exchangers to the ambient. 5.An apparatus for reducing the water and energy consumption of a papermachine with the help of a vacuum system and optimization of solidscontent on the wire section, wherein the apparatus comprises a hybridvacuum system operating in combination with an unfelted, smooth pressroll located above a wire suction roll.
 6. The apparatus of claim 5,wherein said hybrid vacuum system comprises single-stage or two-stageblowers offering their optimal energy efficiency while serving vacuumlocations needing a lower vacuum level, a water ring pump servinglocations requiring a higher vacuum as well as speed-controlledturboblowers and, further, mounted above the wire suction roll of thewire, an unfelted press toll located at the end portion of the vacuumchamber of said wire vacuum roll.
 7. The apparatus of claim 5, whereinthe impellers of blowers are mounted on the pump motor shaft so to allowthe control of their rotation speed.
 8. The apparatus of claim 5,wherein, when there is no water separation provided between the waterring pump and the paper machine, the seal strip lubrication water isused as the seal water of the water ring pump, while the requiredsupplementary water is metered either at a water separator locateddownstream of the water ring pump or by monitoring the pump temperature,whereby the water leaving the water separator is returned to the processor used as the cleaning shower water, and the hot discharge gas exitingfrom the turboblowers is passed via heat exchangers to the ambient. 9.The apparatus of claim 4, wherein said speed-controlled turboblowershave one or two stages and the hot discharge gas exiting from theturboblowers is passed via heat exchangers to the ambient.
 10. Use of ahybrid vacuum system in an optimized fashion to control solids contenton a paper machine wire section.
 11. The use of claim 10, wherein on thewire section of the paper machine is employed a hybrid vacuum systemwith the help of which water removal is firstly carried out at vacuumlocations needing a lower vacuum level and next at locations requiring ahigher vacuum using the vacuum system units at their optimal energyefficiency, while simultaneously, the solids content on the wire sectionis optimized with the help of an unfelted press roll mounted above thewire suction roll.
 12. The use of claim 10, wherein on the wire sectionof the paper machine is employed a hybrid vacuum system, whosesingle-stage or two-stage blowers are used to at their optimal energyefficiency to serve vacuum locations needing a lower vacuum level andwater removal at locations requiring a higher vacuum is carried out withthe help of a water ring pump, while other vacuum locations are servedby speed-controlled turboblowers and, simultaneously, the solids contenton the wire section is optimized by mounting above the wire suction rollan unfelted press roll located at the end portion of the vacuum chamberof the wire vacuum roll.
 13. The method of claim 2, wherein in themethod single-stage or two-stage blowers are most advantageously used toserve vacuum locations needing a lower vacuum level, typically at avacuum of 0-60 kPa, while the higher vacuum level is elevated to a levelmaximized with the help of a water ring pomp to about 70-75 kPa and,additionally, the wire press forces water into the holes of wire suctionroll so as to form water plugs therein thus further enhancing the solidscontent.
 14. The method of claim 3, wherein in the method the seal striplubrication water is used as the seal water of the water ring pump,while the required supplementary water is metered either at a waterseparator located downstream of the water ring pump or by monitoring thepump temperature, whereby the water leaving the water separator isreturned to the process or used as the cleaning shower water, and thehot discharge gas exiting from the turboblowers is passed via heatexchangers to the ambient.
 15. The apparatus of claim 6, wherein theimpellers of blowers are mounted on the pomp motor shaft so to allow thecontrol of their rotation speed.
 16. The apparatus of claim 6, wherein,when there is no water separation provided between the water ring pumpand the paper machine, the seal strip lubrication water is used as theseal water of the water ring pump, while the required supplementarywater is metered either at a water separator located downstream of thewater ring pump or by monitoring the pump temperature, whereby the waterleaving the water separator is returned to the process or used as thecleaning shower water, and the hot discharge gas exiting from theturboblowers is passed via heat exchangers to the ambient.
 17. Theapparatus of claim 5, wherein said speed-controlled turboblowers haveone or two stages and the hot discharge gas exiting from theturboblowers is passed via heat exchangers to the ambient.
 18. The useof claim 11, wherein on the wire section of the paper machine isemployed a hybrid vacuum system, whose single-stage or two-stage blowersare used to at their optimal energy efficiency to serve vacuum locationsneeding a lower vacuum level and water removal at locations requiring ahigher vacuum is carried out with the help of a water ring pump, whileother vacuum locations are served by speed-controlled turboblowers and,simultaneously, the solids content on the wire section is optimized bymounting above the wire suction rod an unfelted press roll located atthe end portion of the vacuum chamber of the wire vacuum roll.